@Thunderforge8 I think you might be experiencing I different issue, more similar to this: Home city resets to 0 level after completing the A Pirates’ Help misionn though it sounds like the issue is no longer occurring?
@MadeMantella469 & @Ziper8941
Would you be able to check your local files and let me know what the exact colosseum scenario is called?
inside of C:\Users\[user]\Games\Age of Empires 3 DE\[0]\Scenario you should see a list of all the scenarios you have downloaded like Colosseum World War.age3scn.
Any further info could help 
Thank you!